A class of phototropic glasses is known which has an index of refraction of 1.523 and a thermal coefficient of expansion of from 45 to 55.times.10.sup.-7 /.degree. C. and which is suitable for use in ophthalmological applications for making spectacle glasses of the single-strength type. Such class of phototropic glasses comprises borosilicate glasses containing certain quantities of silver and halogens which are concentrated in and around phase-separations in such a glass and which phase separations are produced by controlling the temperatures used in the manufacture of such a glass. Such phase-separations, if they are amply supplied with silver and halogen, are the carriers of phototropy. Under actinic radiation, these glasses darken through reversible silver-separation, and, after termination of such insolation, the so-darkened glasses regenerate into their original transparent condition. A more accurate description of these glasses is found, for example, in Gliemeroth and Mader (Angew. Chemie 9 (1970) 434, International Edition in English), "Struktur und Beeinflussung der Phototropie" by Bach and Gliemeroth (Glas-techn. Ber. 44 (1971) 305, or J. Amer. Ceram. Soc. 54, ( 1971), 528.
Another known class of phototropic glass has a characteristic refractive index above 1.60, but this class, unfortunately for multifocal spectacle glass purposes, possesses coefficients of expansion above about 70.times.10.sup.-7 /.degree. C. This class comprises lead-aluminoborate glasses which possess, by reason of their special inclination to disassociation, particularly favorable phototropic characteristics; see German Pat. No. 1,596,847.
In general, the thermal annealing process for producing the disassociation areas which determine the phototropic qualities of such phototropic glasses is dependent upon temperature and time. Thus, after the initial melting process to form the glass, the silver- and halogen- components are present typically in homogeneous solution and these components are concentrated in dissociation areas by means of such a tempering process by using temperatures typically between about 400 and 650.degree. C., the particular times and temperatures employed in any given instance being dependent upon both the type of glass in such dissassociation areas, as well as the thermal history through which a given glass has passed after its melting and before its tempering, during which some sort of germination may be inferred. For the typical phototropic borosilicate glass utilized today predominantly as single-vision-glass or matrix-phase-glass, the temperatures of a tempering process lie between about 550.degree. and 650.degree. C. To achieve timely, constant production of a phototropic glass, it is necessary, in addition to having a glass of uniform composition, to have a constant tempering process. Therefore, upon the completion of melting, it is preferred to reserve glass temperatures above about 400.degree. C. exclusively for the tempering process.
Temperatures above 400.degree. C. are, however, necessary if it is desired to produce multi-strenght (or multi-focal) glasses by fusing together two separately formed starting glasses, as those skilled in the art will appreciate. Because of this need to utilize such high temperatures during such a fusing, phototropic characteristics in a starting glass can be, and typically are, disturbed, so that experts in this art were previously of the opinion that production of phototropic, multi focal glasses by fusing was not possible.
Although phototropic spectacle glasses utilizing single-vision-glasses that means such reinforcing glasses have been produced for some years, the general need for phototropic multifocal spectacle glasses formed of two co-fused phototropic glasses has not been satisfied. Thus, heretofore in this art, to produce bifocal- or multifocal glasses without phototropic characteristics through fusing two types of glasses are selected out of non-phototropic materials, fused onto one another, and subsequently ground and polished.
Also, heretofore, partially phototropic bifocal glasses have been known which are formed of non-phototropic and phototropic glass materials which cannot be fused onto one another, owing to the different respective thermal coefficients of linear expansion. For example, one such partially phototropic spectacle glass made use of a normal (e.g. non-phototropic) multi-strength glass formed into a convex lens surface to which was secured a layer of phototropic glass by means of a polymerized synthetic-material intermediate layer. When such a spectacle glass has good phototropic qualities, it is substantially thicker and heavier than normal spectacle glass. Hence, the thickness of such a phototropic glass laminate is mainly controlled by the phototropic characteristics desired. In a given such spectacle glass, a compromise must be made between the opposing tendencies to increase thickness and weight on the one hand, and to reduce phototropic effect on the other.
In addition, heretofore, phototropic bifocal- or multifocal- glasses, have been known in which both the near portion and the remote portion of the glass are made out of one and the same phototropic glass material, there being different surface curves ground into a given lens formed of such glass to achieve a desired bifocal or multifocal working capacity. In an optically better construction, for example, such a single component, bi-focal glass lens has, between the near- and the remote-portion, a step or shelf which is prominently visible to the onlooker and which interferes with lens polishing. In an optically less good construction, such a single component bi-focal glass lens has a strong picture cycle. For such reasons, and related matters, the number of new multi-strength-glass lenses ground out of such one piece phototropic glass continuously declines.
Further, heretofore, so-called sliding-view glasses made of phototropic material have also been known in which the lenses produced from one piece of phototropic glass, and in which, between the near- and the remote portions, a continuous transition exists. Such glasses have, however, for various reasons been disseminated relatively little.
The most important part of the market for multicomponent spectacles belongs to spectacle glasses formed by fusing a glass of greater index of refraction onto a carrier glass of lower index of refraction. Prior to the present invention, it was not possible to produce such co-fused, phototropic, glasses entirely, or even partially, using phototropic starting glass materials. The great market demand for spectacles made with such co-fused, phototropic glasses could, therefore, not be satisfied.